1. Field of Invention
This invention relates to a combined magnetic/centrifugal-fluid seal for hermetically sealing a rotatable shaft supported in a closely-fitted housing in a non-contacting, non-wearing manner.
More particularly, the invention relates to a hermetic seal that comprises a combined magnetic seal employing a ferrofluid for hermetically sealing a rotatable shaft in the zero and low speed range and which utilizes the ferrofluid to form a centrifugal fluid seal at higher shaft speeds.
2. Background Problem
There are many applications, such as pumps, compressors and the like, where hermetic sealing of a rotatable shaft in a non-contacting, non-wearing manner is not only desirable but essential during both zero or low speed operation and during high speed rotation. Because of the sealing requirement in high speed applications, and in order to reduce wear, it is desirable that the seal be non-contacting in nature.
One well known form of non-contacting seal for use at zero and low speeds is the magnetic ferrofluid seals of the type described in U.S. Pat. No. 2,863,538--issued Dec. 9, 1958 for a "Permanent Magnet Seal"; U.S. Pat. No. 3,746,407--issued July 17, 1973 for a "Ferro Hydrodynamic Low Friction Bearing"; and an article entitled "Magnetic-Fluid Seal" appearing in Machine Design magazine, Mar. 28, 1968 issue, pages 145-150, for example. While known designs of ferrofluid seals can operate satisfactorily at zero speed and low speeds, as in a vacuum chamber feed-through, they tend to break down and leak at high rotational speeds.
A known form of non-contacting seal which can operate satisfactorily at high rotational speeds is the centrifugal seal of the type described in U.S. Pat. No. 3,694,042--issued Sept.26, 1972 for a "Radial Fluid Bearing", for example. Such known centrifugal seals can support a large pressure difference based on the level difference of a pool of fluid formed on the two sides of a rotating disc or ring attached to a shaft. The pool of fluid is both formed and maintained by centrifugal force effects. However, centrifugal seals have little capacity at low rotational speeds and collapse and leak at standstill.
The seals described and illustrated in U.S. Pat. Nos. 4,304,411 (Wilcock et al) and 4,200,296 (Stahl et al) were developed to overcome the aforementioned deficiencies of the magnetic seal at high speed operation and the centrifugal seal at low speed operation. The disclosures in both of these patents in their entirities are expressly incorporated herein by reference. Both of these patents disclose seal arrangements having separate centrifugal and magnetic seal sections of the type described individually above. During high speed operation a centrifugal sealing mode is established whereby magnetically permeable fluid (i.e. ferrofluid) is thrown radially outward by centrifugal force into the centrifugal seal section. The same ferrofluid returns to the magnetic seal section to effect a magnetic seal mode as the operating speed decreases. Sealing at both high speed and low or zero speed is quite effective in both the Wilcock et al and Stahl et al seals. However, some leakage of gas across the seal can occur during transition between sealing modes as the fluid moves from one section to the other.
Another prior art seal also employs separate magnetic and centrifugal seals but utilizes different fluids for each seal. Specifically, during high speed operation the magnetically permeable fluid is thrown by centrifugal force into a toroidal holding tank while a separate fluid, generally an oil, is thrown into the centrifugal seal section. As is the case with the Wilcock et al and the Stahl et al seals, leakage of gas across the seal can occur during transitions between high and low speeds.